Electron discharge devices



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. INVENTORS PALMER R DERBY LAWRENCE L. CLAMPITT AT RNEY United States Patent ELECTRON DISCHARGE DEVICES Lawrence L. Clampitt, Needham, and Palmer P. Derby,

Wellesley, Mass., assignors to Raytheon Manufacturing Company, Waltham, Mass., a corporation of Delaware Application December 18, 1953, Serial No. 399,054

Claims. (Cl. SIS-39.57)

This invention relates generally to magnetron microwave generators and more particularly to improved means for frequency modulating generators of this type.

In its basic form, the conventional magnetron microwave generator comprises a centrally disposed thermionic cathode surrounded by a plurality of radially extending anode cavity resonators tuned to the operating microwave frequency. A source of constant operating potentlal 1s connected between the cathode and anode in a manner whereby the anode is at a positive direct current potential with respect to the cathode. A substantially constant intensity magnetic field is applied axially to the cathodeanode chamber. anode vane tips adjacent to the cathode, resulting in auxiliary electric fields in the anodecavity resonator. These fields interact with the electrons travelling in cycloidal paths from the cathode to the anode, thus abstracting energy from the travelling electrons and producing substantial oscillations at the operating microwave frequency. h

It has been found that if an electron stream is caused to flow through one or more of the cavity resonators in the anode in a direction parallel to the axis of the cathode, the oscillator output will be at a frequency different from what it would be in the absence of such a stream of electrons. Such a stream of electrons can be made -to flow in the desired region by, in effect, forming a diode across one or more of the resonators. This can be done by the use of a supplementary cathode positioned in or below one or more of the resonators or in a separate cavity resonator coupled to one of the main resonators, and applying a voltage between this cathode and the anode. The frequency of oscillation can be made to vary as a function of this applied voltage. This method of tuning is called electronic tuning and is more fully described in chapter 15, volume 6, of Microwave Magnetron of the Radiation Laboratory Series, edited by George B. Collins.

Such an auxiliary diode requires considerable change in the power dissipated in the auxiliary diode to effect any substantial change in the operating frequency of the magnetron. It is expensive to provide a modulating signal of sufiicient power to. cause a substantial change in the operating frequency of most magnetrons.

By the present invention, it is possible to accomplish a relatively large frequency shift with relatively low modulating signal power withv a structure that does not add substantially to the bulk of the magnetron. This is accomplished by arranging an annular auxiliary cathode,

about the main cathode and in a plane perpendicular 2,849,649 Patented Aug. 26, 1958 "ice - The signal voltage and a bias potential are applied between Electrical charges accumulate on the to the axis of the main cathode. The flow of electrons the grid elements and the auxiliary cathode. When a second array of grid elements is used, it is given a positive potential with respect to the auxiliary cathode. The result is that a relatively small amount of signal can cause the desired amount of frequency shift in a substantially linear manner.

Other and further advantages of this invention will be apparent as the description thereof progresses, reference 'being had to the accompanying drawings wherein:

Fig. 1 is a vertical sectional view of a magnetron utilizing the invention; I I

Fig. 2 is an enlarged view of the auxiliary cathode and associated grids shown as a section along the lines 2-2 of Fig. 3; and

Fig. 3 is a top view of the auxiliary cathode and grid sub-assembly.

In Fig. 1, the reference numeral 10 designates the cavity resonators enclosure of a magnetron having an anode comprising a ring 11 of conductive material such as copper supporting a plurality of radially disposed vanes 12 that form with ring 11 a plurality of cavity resonators. The enclosure 10 is closed by a top plate 13 and a bottom plate 14 also of conductive material. The top plate 13 is formed with an opening into which is fitted a pole piece 1.5. The bottom plate 14 is also formed with an opening into which is fitted a second pole piece 16. A cathode 17 is positioned within an opening 18 in the lower pole piece 16 with its axis at the center of the anode vanes 12 and perpendicular to them. The cathode 17 is supported in this opening by a cathode support 20 of any of the types customarily used in magnetrons that serve to support the cathode and connect it to the external circuit without affecting the vacuum within the cavity. A choke shield 21 is mounted about the cathode support 20 within the opening 18. This shield is formed as a sleeve of conductive material supported about and concentric with the cathode support '20 by means of an annular enlargement 21a at a point a quarter of a wave at the operating frequency of the magnetron from the cathode, for a purpose to be explained later. Alternating vanes 12 are connected by straps 22 and 23 mounted in notches 24 formed in the vanes 12 in a manner well known in the art. A supplementary cathode 25 in the form of a ring is mounted about the shield 21 by a ceramic ring 26. The details of this supplementary cathode are best shown in Figs. 2 and 3.

An annular heater element 27 for the cathode 25 is supported below the cathode by a doughnut-shaped housing 28. Two grid supporting rings 30 and Marc mounted about the shield 21 just above the auxiliary cathode 25. These grid rings 30 and 31 have attached to them several radial wires 32. The rings 30 and 31 and the cathode 25 are formed with mounting lugs 34a, 34b and 340, respectively, as best seen in Fig. 3. The lugs 34a and 34b are joined to the rings 30 and 31 by bowed loops 35 of metal to permit thermal expansion without distortion of the grids.

These grid rings 30 and 31 are separated from the auxiliary cathode 25 by ceramic buttons 36 set in openings 37'in.the cathode 25. The grid ring 31 is mounted above the first ring 30 on a second ceramic ring 38 attached to the sleeve 21 and separated from the lower ring 30 by a ceramic ring 40. The ring 30 is attached to the bottom plate 14 by screws 41 and insulated sleeves 42. The ring 31 is attached to the bottom plate 14 by screws 43 and insulated sleeves 43a. The cathode 25 is attached to the bottom plate 14 by screws 44 set in insulating sleeves 45.

The heater 27 is connected to a source of potential 46 through conductors 47 and 48 passing through the plate 14 by means of insulators 50 and 51. The supplementary cathode is connected to a source of negative potential 52 through conductors 53 set in insulator 54. The first grid is connected to a source of biasing potential 55 through conductor 56 in insulator 57 and is also coupled to the output of a source of modulating signal 58 through a capacitor 60. The output of the modulator 58 is also coupled to the cathode 25 through a capacitor 61, the upper grid 31 being connected to a source 62 of positive potential through conductor 63 set in insulator 64 in the lower plate 14.

In operation, the cathode 25 emits a stream of electrons that pass through the resonators near the inner ends of the vanes 12 to the upper plate 13. The presence of this beam of electrons in each cavity resonator changes the effective capacity of the cavity resonator and thus its resonant frequency and the operating frequency of the magnetron oscillator. The resulting frequency shift is a function of the amplitude of the beam current. The effect of the grids 30 and 31 is to control the amplitude of the beam current in much the same way as the control and screen grids control the electrons reaching the plate in a conventional tetrode. Theresult is that the frequency of the output of the magnetron oscillator is varied in accordance with the output of the modulator. It has been found that this shift in frequency can be made linear over a useful band of frequency with a minimum of modulating power. The high impedance presented by the grid to auxiliary cathode circuit to the modulator reduces the modulating power required to effect a given amount of modulation. In a representative type of magnetron oscillator, electron tuned by a diode arrangement with a similar halo cathode, a shift in frequency of 0.05 megacycle per second was obtained for each volt of modulating po-- tential applied. With the use of the grid structure of this invention, the frequency was shifted 0.2 megacycle per second for each volt of modulating potential applied. The shield 21 serves the purpose of preventing electrostatic interaction between the two cathode structures.

This completes the description of the particular embodiments of the device disclosed herein. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. Accordingly, it is desired that this invention be not limited by the particular details of the embodiments described herein except as defined by the appended claims.

What is claimed is:

1. In an electron discharge device having an anode defining a plurality of cavity resonators and a principal cathode formed on a cylindrical surface, a supplementary cathode disposed on a plane perpendicular to the axis of the principal cathode and parallel to the plane of the resonators formed in the anode, and a plurality of grid electrodes extending radially about the axis of the principal cathode in a plane perpendicular to said axis.

2. In an electron discharge device having an anode defining a plurality of cavity resonators and a principal cathode formed on a cylindrical surface, a supplementary cathode disposed on a plane perpendicular to the axis of the principal cathode and parallel to the plane of the resonators formed in the anode, and a plurality of grid electrodes extending radially about the axis of the principal cathode arranged in two planes perpendicular to said axis.

3. In an electron discharge device having an anode defining a plurality of cavity resonators and a principal cathode formed on a cylindrical surface, a supplementary cathode disposed on a plane perpendicular to the axis of the principal cathode and parallel 'to the plane of the resonators formed in the anode, and a plurality of grid electrodes extendingradially about the axis of the,

principal cathode arranged in two planes perpendicular to said axis, the electrodes in one plane being directly below those in the second plane.

4. In an electron discharge device having an anode defining a plurality of cavity resonators and a principal cathode formed on a cylindrical surface, a supplementary cathode disposed on a plane perpendicular to the axis of the principal cathode and parallel 'to the plane of the resonators formed in the anode, and a plurality of grid electrodes extending radially about the axis of the principal cathode arranged in two planes perpendicular to said axis, the electrodes in one plane being insulated from those in the second plane and from the cathodes.

5. In an electron discharge device having an anode defining a plurality of cavity resonators and a principal cathode formed on a cylindrical surface, a supplementary cathode disposed on a plane perpendicular to the axis of the principal cathode and parallel to the plane of the resonators formed in the anode, a plurality of grid electrodes extending radially about the axis of the principal cathode and arranged in two planes perpendicular to said axis, the electrodes in one plane being insulated from those in the second the electrodes in one in the second plane.

plane and from the cathodes, plane being directly below those References Cited in the file of this patent UNITED STATES PATENTS 2,392,380 Varian Jan. 8, 1946 2,406,370 Hansen et a1 Aug. 27, 1946 2,414,785 Harrison et al. Ian. 21, 1947 2,468,243 Spencer Apr. 26, 1949 2,508,473 Shoupp May 23, 1950 2,538,597 Steele Jan. 16, 1951 2,542,797 Cuccia Feb. 20, 1951 2,585,741 Clogston Feb. 12, 1952 2,602,156 Donal et al. July 1, 1952 2,810,856 Reed et al. Oct. 22, 1957 

